When will the iPhone 7 be released? And what are the features that will be added to it that don't exist in iPhone 6?

The iPhone 7 will be released On September 2016.

The new iPhone will be announced in September. I say this as majority of the previous iPhone releases have been on that month. The IPhone will be more focused towards the display as it did with the 6s. There could be potential improvements with the force touch and a new additional feature. There have also been rumors that the headphone jack on the IPhone 7 will be ditched and headphones will have to be connected through the lightning port.

I think it will be announced in November or October, & apple is giving very little upgrades each version so I think a better camera with better overall performance & new design with better battery life will be the upgrades

As always, In september and certain feature will remain secret until the night of press conference.  

I think that the iPhone 7 invades markets starting next year

Apple didn't announce anything about iphone 7, and all what is found on the web are journalists´ previsions and rumours, the last released device is iPhone SE.